Angela Milner discusses the fossil record

Acanthostega is a transitional fossil that shows the evolution of fish into tetrapods, 4-legged animals that lived on land. Its fins had turned into limbs and it had a strong rib cage capable of supporting its body outside of water.

Ida, Darwinius masillae, has an important place in the fossil record as the most complete fossil primate ever discovered.

Dr Angela Milner, fossil expert at the Museum, discusses Ida's significance, and explains some of the key developments in the fossil record in the history of life.

Research interests

Dr Angela Milner's main areas of work are on theropoda (the meat-eating dinosaurs) and the origin of birds. After describing the new species Baryonyx in 1986, Angela continues to research spinosaurs, the fish-eating theropods.

Angela led the research team that used CT scans to demonstrate that Archaeopteryx had a bird-like brain and an inner ear just like modern birds, confirming that it could fly. Next, Angela worked on brain evolution in more recent fossil birds from the Eocene of southern England, in collaboration with Dr Stig Walsh at the National Museum of Scotland.

Angela's other research involves studying much older fossils - tetrapods from the Carboniferous Period 325 - 320 million years ago.

The fossil record

Fossils are the remains of long-dead organisms that are preserved in sedimentary rocks and they hold the clues to the past life on Earth.

The fossil record extends back to between 3.8 and 3.46 billion years ago. The oldest fossils are microscopic bacteria, or bacteria-like organisms. They are so difficult to study that there is some controversy about whether the very earliest fossils really are traces of organic life.

Gaps in the fossil record

We are still discovering new species in the fossil record and that’s what makes palaeontology (the study of fossils) so fascinating.

The first 85% of life's history was dominated by fossil microbes. The fossil record becomes increasingly rare in older and older rocks. This is because soft tissue rots quickly after death and there is little chance of it being preserved. So, it is more difficult to find fossils of, for example, single-celled organisms than of mammals, whose bones and teeth are more likely to be fossilised.

Some ecosystems are also very poorly represented in the fossil record. There is very little information about life in upland and mountain areas that lack high levels of sedimentation, and what happened in the open oceans and deep sea is largely a mystery. The abyssal plain, past or present, rarely reaches the surface.

Studying fossils

Scientists have access to sophisticated new techniques for studying and analysing fossils that weren’t available in the past.

For example, CT scanning is a process that allows scientists to take a 3D scan of hard tissues. This lets them find out, for example, what is happening inside the skulls of animals, rather than looking only at the external features of the bones.

Exciting discoveries

One of the most exciting discoveries in Britain took place in 1983 when a fish-eating dinosaur, Baryonyx, was discovered by an amateur collector just 30 miles south of London.

Since the beginning of the nineteenth century, dinosaurs had been collected from 135 – 125 million year old rocks that underlie much of southern England, but this was something entirely new.

Timeline

3.8 – 3.46 billion years ago: Microscopic bacteria, or bacteria-like organisms lived in the shallow seas. These make up the first 85% of the history of life.

1.2 billion years ago: Simple algae (single-celled plants) evolved.

630 million years ago: The first complex multicellular organisms appear in the fossil record, although they may have begun to exist long before this time.

525 – 520 million years ago: The Cambrian explosion saw the first appearance in the fossil record of the body plans that would later form the basis of modern animals. These include very primitive jawless fish-like animals, the ancestors of modern vertebrates (animals with a backbone).

375 – 365 million years ago: Fish were evolving into the first back-boned land-animals or tetrapods (animals with 4 limbs).

300 million years ago: Reptiles appeared at about this time. They were the first back-boned animals that did not need to go back into the water to breed. The waterproof egg had evolved.

228 – 65 million years ago: Dinosaurs dominated the land during the Mesozoic Era.

200 million years ago: The first mammals appeared. They were the size of shrews. Mammals did not diversify and become dominant until after the last of the non-avian dinosaurs became extinct.

147 million years ago: The earliest bird, Archaeopteryx, appeared.

47 million years ago: Ida, Darwinius masillae, an early primate, lived in tropical rainforest in what is now Germany.

4 million years ago: The earliest hominins, the group that includes all human species including ourselves, were established. They may go back even further in time.

200,000 years ago: The first anatomically modern humans appeared in Africa.

Transitional forms

Fossils can provide a record of structural innovation, or significant steps in the evolution of new features. Such discoveries are transitional forms.

The popular media tends to use the term ‘missing link’ to describe transitional forms although it is inaccurate and confusing. In reality, the discovery of more and more transitional fossils continues to add to knowledge of evolutionary transitions although such fossils are rarely, if ever, on the direct line of evolution from one major group to another.

Moving onto land

Recent discoveries have given us a much better understanding of a crucial stage in our own evolution, the emergence of vertebrates onto land.

Several intermediates between fishes and amphibians have been discovered in Late Devonian river sediments between 375 and 365 million years old. Tiktaalik and Acanthostega show key structural stages in the transition from water to land.

One example is the modification of fins to limbs, complete with the same bones we have in our arms (humerus, ulna and radius), wrists and fingers. Another is the robust rib-cages showing that they were weight-bearing, able to support their bodies out of water.

Both animals have their own peculiarities suggesting that they are not an earliest ancestor to any living animal, but they serve as proof that intermediates between very different types of vertebrates existed and illustrate how the adaptations to life on land evolved.

From dinosaurs to birds

Just as Tiktaalik and Acanthostega are keys to the origin of tetrapods, Archaeopteryx is a key to the origin of birds.

Archaeopteryx is probably the best known transitional fossil. It lived 147 million years ago at the end of the Jurassic period.

Archaeopteryx was found in a very fine-grained limestone that preserved not only its skeleton, essentially that of a small meat-eating dinosaur, but also the very clear imprints of wings and flight feathers almost exactly like those of modern birds. The feathers are dramatic evidence of characters that it shares exclusively with all other birds.

Ida, Darwinius masillae

Messel, a volcanic crater lake bed near Frankfurt in Germany, preserves a rich tropical ecosystem that existed 47 million years ago. Among the fauna is the newly-described early primate Darwinius masillae, nicknamed Ida.

Ida shares primitive features in common with other similar early primates. However, it has not yet been demonstrated that Ida shared any advanced features to link it specifically with anthropoids, the primate group that includes monkeys, apes and us. Not all fossils, however well-preserved, turn out to be key transitional forms.